It has long been realized that motionless mixers if made to work efficiently, provide certain economic advantages over dynamic mixers for, as the name implies, motionless mixers employ no moving parts. As such, motionless devices are generally less expensive to configure and certainly much less expensive to maintain while providing the user with an extended useful life for the mixer product in service.
Prior art approaches to motionless mixers have generally involved expensive machining, molding, casting or other fabrication of the component mixer elements coupled with some type of permanent attachment between elements and a conduit and/or between elements within a conduit. The resulting cost and difficulty of manufacture results in a relatively expensive end product. Moreover, many of the prior mixers provide less than complete mixing particularly with respect to material flowing along the walls of the conduit. This so-called "wall-smearing" is related to the parabolic velocity profile of a fluid having laminar flow in a pipe where the fluid velocity is small or zero along the wall surfaces.
Despite their limitations, static or motionless mixers are in common use in many industrial fields and are applied to both laminar and turbulent flow applications. A wide variety of mixing element designs are available from different manufacturers. Mixing elements are installed in a tube or pipe conduit in series and are fixed in position relative to the conduit wall. The cross-section is usually round but can be square or even rectangular. Materials introduced to the inlet or upstream side of the conduit on a continuous flow basis emerge mixed.
The number of mixing elements required to complete a given mixing task can range from two to twenty or more depending on the difficulty of the mixing application. In general, more mixing elements are required to solve laminar flow mixing problems than are needed in turbulent flow situations. One of the most difficult laminar flow mixing problems, for example, is to mix a small quantity of a low viscosity additive into a much higher viscosity main product flow. Mixing involves the application of the principals of distribution and dispersion.
Referring to FIG. 1, it is seen that a small amount of an additive "A" is introduced on a continuous flow basis to a continuous main product flow "B". The two components then pass through a mixing system. The additive "A" is divided into many small components by the mixing system and the stream exits with the additive distributed across the cross-section of the main flow "B". The typical distance "S" between the concentration centers of the additive is small relative to the main flow diameter "D". Good distribution of additive "A" in stream "B" has been achieved.
The concept of dispersion is shown in FIGS. 2A and 2B. In FIG. 2A, the additive "A" is distributed in the main flow stream "B" material where molecular diffusion between "A" and "B" is virtually zero. The concentration values are either 0% or 100% or, in other words, the intensity "I" of "A" and "B" has a value of either 0% or 100%. In other words, zero dispersion has been achieved. However, in FIG. 2B some degree of molecular diffusion has occurred and the range of the intensity value found in the flow stream as measurements are taken across the conduit is now less than 0% to 100%.
It is obviously a goal in any mixing device to improve distribution and dispersion of component fluid streams. However, this is oftentimes difficult if this goal is attempted by simply adding more mixing elements. The addition of mixing elements often results in pressure drops across the mixing system while such systems tend to increase in length and cost to a point where such parameters prove prohibitive. Furthermore, small filament streams of component "A" can oftentimes tunnel through the mixing structure without further reduction in size.
In 1975, Komax Systems revolutionized the field of mixing by the invention disclosed in its U.S. Pat. No. 3,923,288. The '288 patent disclosed a stationary material mixing apparatus which, as shown in FIG. 3, included a conduit 2 having an internal chamber 4 in which a plurality of elements 6 and 8 are fitted. Element 6 was shown to include a central flat portion 10, the plane of which was intended to be generally aligned with the longitudinal axis of chamber 4. First and second ears 12 and 14, rounded or otherwise configured at their outside peripheries for general fit to the wall of chamber 4, were bent upward and downward from the flat portion 10. A second pair of ears 16 and 18 at the opposite side of flat portion 10 were bent downward and upward, respectively. The outside peripheral edges of ears 16 and 18 were also shown to be rounded or otherwise configured for general fit to the wall of chamber 4. Elements 6 and 8 were formed from a single flat sheet by a punch press. The angle .lambda. between ears 12-14, 16-18, 22- 24 and 26-28 was taught to be preferably in the range of about 30.degree. to 120.degree..
When a pair of elements 6 and 8 were nested together, the axially overlapping portions of the elements where the ears mesh defined what can be termed a "mixing matrix" zone where the longitudinally moving material has counter-rotating velocity vectors induced thereon with simultaneous inward and outward radial vectors. These complex mutually opposed angular and radial vectors result in mutual shearing effects which cause the materials to mix and recombine in a different configuration subsequent to the mixing matrix as the materials flow briefly past the flat central region (10, 20) of the elements 6 or 8. This flat central region (10, 20) or non-axially overlapping length of the element 6,8 has been found to contribute significantly to the successful operation of the invention disclosed in the '288 patent.
It is known that materials proceeding under laminar flow are particularly difficult to mix as a parabolic velocity distribution profile appears across the pipe or conduit. It takes a finite time or pipe length to establish this profile for what is called fully developed flow. If L.sub.e is this length and D is the pipe diameter and R.sub.e is the Reynolds number, then: EQU L.sub.e /D=0.035R.sub.e
In mixing plastics, one experiences extremely low Reynolds numbers, usually less than 0.01. As such, the length taken to establish the final laminar profile is very short compared to the pipe diameter. This means extensive mixing is given an opportunity to occur during the residence time in the element flat region. Although the invention disclosed in the '288 patent was a marked advance over the prior motionless mixing devices known at the time, it has been found that improvement could be made to this design to improve the mixing operation particularly when laminar flow conditions were involved.